The present invention relates to a controller for variable displacement compressors to control displacement.
A refrigeration circuit of a typical vehicle air-conditioning system includes a condenser, an expansion valve, which functions as a depressurizing device, an evaporator and a compressor. The compressor draws refrigerant gas from the evaporator and compresses the gas. The compressor then discharges the gas to the condenser. The evaporator transfers heat between the refrigerant in the circuit and air in the passenger compartment. Heat from air that flows about the evaporator is transferred to the refrigerant flowing through the evaporator in accordance with the thermal load or the cooling load. The pressure of the refrigerant gas at the outlet of the evaporator represents the magnitude of the thermal load.
A vehicle variable displacement swash plate type compressor has a displacement control mechanism for setting the pressure (suction pressure Ps) in the vicinity of the outlet of the evaporator to a predetermined target suction pressure. The mechanism adjusts the compressor displacement by changing the inclination angle of the swash plate such that the flow rate of refrigerant corresponds to the cooling load. The displacement control mechanism has a control valve. The control valve includes a pressure sensing member, which is a bellows or a diaphragm. The suction pressure is detected by the Ps pressure sensing member. A valve opening is adjusted in accordance with the displacement of the pressure sensing member, which changes the pressure in a crank chamber, or crank pressure Pc.
A simple control valve that imposes a single target suction pressure cannot control the air conditioning performance accurately. Therefore, an electromagnetic control valve that changes a target suction pressure in accordance with an external current has been proposed. Such a control valve includes an electromagnetic actuator such as a solenoid. The actuator changes a force acting on the pressure sensing member in accordance with an external current to adjust the target suction pressure.
According to the above-described control method, however, even if the target suction pressure is changed by electric control, the actual suction pressure may not reach the target suction pressure spontaneously. That is, the cooling load is likely to affect whether or not the actual suction pressure well responds to a change in the target suction pressure. It is not therefore possible to promptly and reliably alter the displacement of a compressor even if the actual suction pressure is regulated as needed by electric control.
Accordingly, it is an object of the present invention to provide a control apparatus for a variable displacement type compressor, which can promptly and reliably change the displacement of the compressor.
To achieve the above objective, the present invention provides a controller for controlling the displacement of a compressor. The compressor is included in a refrigeration circuit of an air conditioning system. The controller comprises a pressure difference detector. The pressure difference detector detects the pressure difference between two pressure monitoring points located in the refrigeration circuit. An air conditioning switch turns the air conditioning system on. A temperature detector detects the temperature in a compartment. A mechanism controls the displacement of the compressor such that the pressure difference detected by the pressure difference detector approaches a target value. A computer determines the target value. The computer changes the target value, depending on the detected temperature. The computer limits the target value to a target value limit. After a predetermined time expires from when the air conditioning switch is turned on, the computer changes the limit.
To achieve the above objective, the present invention also provides a method for controlling the displacement of a compressor installed in a refrigerant circuit of an air-conditioning system. The method comprises detecting the pressure difference between two pressure monitoring points located in the refrigerant circuit, detecting the temperature in a compartment, determining a target value of the pressure difference. The determining includes limiting the target value to a target value limit, changing the target value depending on the detected temperature, changing the target value limit after a predetermined time expires from when the air conditioning system is activated, and controlling the displacement of the compressor such that the pressure difference detected by the pressure difference detector approaches the target value.